The kraft process is generally the most widely used chemical pulping process employed in the paper-making industry. In this process, wood chips or other fibrous materials are heated under pressure with steam in a sodium hydroxide and sodium sulfide solution. The resulting pulp is generally used to make brown paper. It can also be bleached to produce white pulp for writing and other higher grade papers.
The filtrate and washings from the pulping process--the so-called black liquor--generally contain dissolved organic materials from the wood as well as spent pulping chemicals. The black liquor is normally concentrated in evaporators and then burned under both oxidizing and reducing conditions in a boiler or combustor. The combustion of the organic matter contained in the black liquor produces high pressure steam which can be used to generate electricity. The inorganic materials form a molten kraft smelt on the bottom of the combustor. This kraft smelt generally contains sodium carbonate, sodium sulfide, and sodium sulfate. The molten kraft smelt, generally at a temperature of about 1400.degree. to 1550.degree. F., is normally removed from the bottom the combustor via a water-cooled trough (the so-called smelt spout) and fed directly into an aqueous solution contained in a dissolving tank. Thus, the kraft smelt is fed into the dissolving tank while still in the molten state. The kraft smelt dissolves in the aqueous solution and produces a so-called green liquor. The green liquor, after filtering or clarifying to remove undissolved materials, is treated with lime to convert the sodium carbonate into sodium hydroxide through slaking and causticizing reactions. In the slaking reaction, calcium oxide is converted to calcium hydroxide; in the causticizing reaction, the calcium hydroxide reacts with sodium carbonate to produce calcium carbonate and sodium hydroxide. This lime-treatment process produces kraft white liquor containing the pulping chemicals which can then be reused or recycled in the kraft pulping system.
This chemical recovery system forms an important and vital part of the paper-making process. Indeed, without the chemical recovery system, the kraft system would be prohibitively expensive. Moreover, the disposal of the black liquor would likely be impossible in an environmentally acceptable manner. This chemical recovery process or system has, however, a number of problems and/or limitations. For example, feeding molten kraft smelt (normally about 1400.degree. F. or higher) directly into an aqueous solution can result in, and often has resulted in, explosions in the dissolving tank. Such explosions, in addition to potentially damaging equipment and injuring personnel, can release significant amounts of sulfur-containing pollutants into the environment, especially into the atmosphere. In addition, this process results in a significant energy loss due to loss of atmospheric steam escaping from the dissolving tank. Moreover, it is difficult to control the green liquor concentration when feeding molten kraft smelt. Finally, emissions of sulfur-containing pollutants from the dissolving tank, even in the absence of smelt/water explosions, are significantly higher than desired.
Considerable efforts have been made to improve the existing kraft recovery process or to provide alternative kraft recovery processes. See, for example, Nishizawa et al., "Chemical Recovery Process by Direct Carbonation of Smelt," Proc. IUPAC/EUCEPA Symposium on Recovery of Pulping Chemicals (Helsinki) 659-73 (1968); Grace, "Gasification: Route to the Promised Land?'", 70 PIMA 75-76 (1988); Empie, "Alternative Kraft Recovery Processes," 74 Tappi J. 272-76 (1991); DeNovo et al., U.S. Pat. No. 4,303,496 (Dec. 1, 1981); Empie, U.S. Pat. No. 4,441,959 (Apr. 10, 1984); Feldmann, U.S. Pat. No. 4,522,685 (Jun. 11, 1985); and Empie, U.S. Pat. No. 4,526,760 (Jul. 2, 1985). Although the overall process has been improved in a number of ways, the kraft recovery process generally employed today still involves feeding molten kraft smelt directly into an aqueous solution in the dissolving tank.
It would be desirable, therefore, to provide a kraft recovery system which avoids direct contact of the molten kraft smelt with water. Such a system would have a significantly reduced risk of smelt/water explosions. It would also be desirable to provide a kraft recovery system with significantly increased energy efficiencies. It would also be desirable to provide a kraft recovery system with significantly reduced levels of sulfur-laden emissions. It would also be desirable to provide a kraft recovery system which allows better and more reproducible control of the green liquor concentration. The present invention provides such a kraft recovery system. Moreover, the kraft recovery system provided by the present invention can be incorporated into existing kraft recovery systems with relative ease.